Although natural wood as a material of construction has many desirable properties, some of its shortcomings, such as its lack of strength in thin layers, and its inherent combustibility, have restricted the scope of its application. The economic necessity of utilizing "waste" subproducts, and the notable product improvement in strength and appearance has led to the widespread use of laminated wood products in the form of plywood, wood panelling, and veneers, such as doorskin, as well as veneer-covered compressed particle and waferboard. Construction and industrial plywoods are widely used for floor, wall and roof sheathing; decorative hardwood plywood is used for interior panelling as well as for furniture and many specialty applications. The 1984 edition of "U.S. Industrial Outlook" reported that the market for wood panel products in 1983 reached $6.6 billion, of which the industrial softwood panels represented 65% of the shipments; hardwood panels represented 25%, and particleboard held 10% of the market. 2.60 billion square feet (surface measure) of hardwood plywood was produced in the U.S.A. in 1983, versus 3.9 billion square feet in 1977 (a reduction of 33%), while an additional 1.48 billion square feet were imported. (Softwood plywood volumes far exceed those of hardwood plywood; viz. over 20 billion square feet per year.)
If one of the major drawbacks to the use of plywood--its flammability--could be economically counteracted without necessitating the use of claddings, fire resistive gypsum wallboard shielding, space isolation, etc. now required by construction codes, the market and applications for plywood would be further extended.
Plywood and panelling is manufactured by bonding together layers (plies) of thin sheets of wood (veneer). The layers are glued together with the grain direction of adjacent layers at right angles. The veneer is usually roatary-peeled from logs, but may also be sliced or sawn. After trimming, drying and grading, the veneers go to glue spreaders, where adhesive is applied and the plywood panel is laid up. The plywood is generally hot-pressed in large multiopening heated hydraulic presses. The application of both heat and pressure cures the glue.
Construction and industrial softwood plywood is made in two types: Interior and Exterior, on the basis of resistance of the panels to moisture, and in thicknesses ranging from 1/4" to 11/8". Decorative hardwood plywood is available in thicknesses from 1/8" to over 2". The standard size of a plywood sheet is 4'.times.8', but other sizes and thicknesses can be manufactured on special order. Typical plywoods are manufactured in 3 to 5 layers, but with 3 to 6 plies. Although weights will vary with the species of wood used, 5/16" thick unsanded panels weigh approximately 1.0 pound per square foot; 3/4" panels weigh 2.2 pounds per square foot; and 1" thick panels weigh 3.0 pounds per square foot. Hardwood plywood is generally a decorative wall paneling product, but is also used structurally, for example, in the structural integrity of mobile and manufactured homes.
Because of plywood's cross-laminated structure, it can support highly concentrated loads, even at unsupported edges. Shear strength varies with the kind of glue used, even though the glue line is very thin--a few mils thick (dry).
Several types of water-dispersable glues are used in plywood bonding, which vary in their resistance to moisture and are one of the bases used for grading as exterior or interior grade. Most glues used are thermosetting, and these vary in the temperature, temperature-time, and temperature-time-pressure required to "cure", as well as "pot-life", "open time", viscosity, "spreadability", and other parameters essential for a suitable bonding agent. Softwood plywoods generally use phenolic adhesives (phenol-formaldehyde); decorative hardwood plywoods generally use urea formaldehyde resins. The phenolic glues, which are highly water-resistant, will cure in 4 to 5 minutes at temperatures as low as 300.degree. F., but they have a short shelf life. The urea formaldehyde adhesives can be catalyzed with ammonium sulphate at 0.5 to 1.0% to bring about a lower temperature cure, and typically require from 150 to 300 psi pressure (depending on the density of the wood species being bonded) at 230.degree. to 260.degree. F. for 3 to 5 minutes. Resorcinol and phenol-rersorcinol resins are highly waterproof, and if properly catalyzed do not require heating for curing--that is, they bond at room temperatures. Paraformaldehyde is a commonly-used catalyst in bringing about the cross-linking reaction. However, in comparison to the phenolics and ureas, they are very expensive, and hence are not used in plywood or panelling lamination, but are used in laminating heavy timbers and beams ("Glulam"). Other widely used laminating adhesives are based on melamines (melamine formaldehyde, melamine urea, etc.), but these are used mainly in furniture decorative laminates, etc.
Most of the glues used are flammable, although some are "self-extinguishing". They impart little fire resistance to the plywood.
The fire hazard associated with the flammability of wood products is the principal limiting factor in the use of laminated woods as a material of construction. Almost all Local, State, and Regional governing bodies have enacted building Codes which regulate the use of wood products in construction, and which dictate other norms for the prevention, early detection, and extinguishment of fires. (Most "advanced" countries also have national building codes.) Some fire-preventive methods are "structural", i.e. isolation of the wood members behind fire barriers; other are "spatial"--separating wood members by "fire breaks"; and still others require the use of sprinkler or water deluge system installations. "Chemical" methods are also available, although not as widely used. These fall into two major categories: (a) Impregnation with fire retardants; or (b) Coating with paints which retard the spread of flame. Both are effective for short periods of time in reducing flame spread, but neither is significantly effective in increasing the resistance of wood to degradation under sustained fire exposure, or in preventing reduction of its load-bearing capacity.
The National Fire Protection Association (NFPA) has established a Standard (#703-1979) for both "Fire Retardant Impregnated Wood" and "Fire Retardant Coatings for Building Materials". The American Wood Preservers Association has a Standard (AWPA C-27) with a requirement for a flamespread of 25 or less, with no evidence of significant progressive combustion when tested for 10 minutes duration under the ASTM E-84 Test Method. "Fire Retardant Treatment of Lumber and Plywood" is also covered by Federal Specification MIL-L-19140 D (Navy). U.S. Federal Specification SS-A-118(b) covers the use of flame retardant coatings on unfinished interior plywood and other construction products.
Impregnation with fire retardant chemicals is an expensive procedure and a complicated process, since, in order to be effective, the chemicals must be applied under pressure in a closed pressure cylinder which requires that a vacuum be pulled first on the load to extract the air from the wood cells prior to pressurization. Chemicals commonly used in this impregnation are water-soluble salts, such as monobasic and dibasic ammonium phosphates, ammonium sulphate, borax, boric acid, zinc chloride, sodium dichromate and combinations thereof. Some of the salts are corrosive to metal fasteners and hardware (nails, screws, bolts, hinges, etc.); some prevent surface painting; some species of wood must be scored to ensure penetration of the treating solution; and being water-soluble, the salts tend to leach out if exposed to weather. American Plywood Association (APA) mandates that allowable stresses be reduced 1/6th, and the modules of elasticity be reduced 1/10th for fire-retardant chemical pressure-impregnated plywood below normal design standards for untreated plywood. In addition, the fire retardant treatment of wood does not prevent the wood from decomposing and charring under fire exposure. The rate of fire penetration through treated wood is approximately the same as for untreated wood.
Fire retardant coatings are also used to reduce the surface flammability of wood and laminated wood products. Prior Art disclosed four major types of fire-protective paints: (1) ablative; (2) intumescent; (3) sublimating and gas- or vapor-producing; and (4) "ceramic", or inorganic. Ablative coatings are generally thick and heavy and costly, thus limiting their application to special uses. Intumescent coatings are generally soft and easily abraded, and have limited duration of protection--10 to 15 minutes--when exposed to fire before the delicate char cracks and is eroded away, leaving the substrate unprotected. They cannot be used internally ("submerged") between the wood plies since their mode of action is inhibited by the glue line bond strength. The adhesive prevents their normal 10 times (or greater) expansion. Sublimative or gas-producing coatings have even shorter duration of protection--after their snuffing gasses have been produced and wafted away, they are left without a protective mechanism. They have found their major use in the treatment of fibers and clothing where only short-term exposure to flame is a major requirement. Ceramic coatings, as the name implies, require high temperature curing in order to form the ceramic bond, which many structural and building wood components cannot withstand. Inorganic coatings, such as Portland Cement, gypsum, calcium aluminate cement, phosphate-bonded cements, silicate coatings, high temperature silicones, and magnesium "oxychloride" and magnesium "oxysulphate" cement coatings have all been proposed.
H. Ellis, in a companion U.S. Letters Patent Application, now pending, entitled "FIRE BARRIER: I. COATINGS", of which this disclosure is an extension, in part, discloses seven type formulations for inorganic coatings, consisting of synergistic combinations of two and three non-combustible inorganic bonding systems which efficiently prevent flame spread on the surface of susceptible substrates, and at the same time prevent flame from penetrating through the substrate.
The American Society for Testing Materials (ASTM) has issued a series of Standard Test Methods for evaluating the protective abilities of fire retardant coatings and constructions (E-119, "Fire Tests of Building Construction and Materials"; D-3806, "Small Scale Evaluation of Fire Retardant Paints (2-Foot Tunnel Method)"; E-286, "Surface Flammability of Building Materials Using an 8-Foot Tunnel Furnace".) E-84, a 25-Foot "Tunnel" test (the most widely accepted test), has its counterpart in Underwriters' Laboratories (UL) #723; and the National Fire Protection Association (NFPA) #25 test methods. The tests determine the protection a coating affords its substrate; and the comparative burning characteristics of coatings by evaluating flame spread over the surface when ignited under controlled laboratory conditions. The different tests are not comparable, although the results are correlatable. None of them describe or apprise the fire hazard of assemblies under actual fire conditions, but the results are usable as an element in fire-risk assessment.
The tests also give the basis for classifying the performance of the coatings based on their Flame Spread Index Values into Class A, (or I), Class B (or II), and Class C (or III), used by most Building Codes. Index value classification depends on flame spread of the candidate fire retardant coated products or materials of construction, in comparison to known standards. Asbestos-cement board is arbitrarily rated at 0 flame spread; Red Oak lumber is rate at 100. Materials with the lowest rate of flame spread (0-25) are classified as Class I, and are permitted in areas where fire hazard is most severe (such as exitways of unsprinklered buildings); materials with rates from 26 to 75 are Class II, and are permitted in areas of intermediate severity; materials rated 76 to 200 (such as PS-1 uncoated construction plywood) are Class III, and are permitted in most rooms when suitably protected by other design factors. Hospitals, institutions, and buildings of public gatherings require Class I products or fire-protective barriers. There is also a Class D (or IV), with Index Values of from 201 to 500, and a Class E (or V), with Index Values of over 500.
Most untreated softwood plywoods and decorative hardwood plywoods fall into Class III, but this varies, depending on wood species, thickness, and glue type. These types of plywood constitute by far the great majority of laminated wood boards. Therefore, they are accepted for interior finish only, and not for exitways. If they could be upgraded to Class I economically, the range of their potential application would be extended considerably. The market for these products would also be considerably enlarged by the resulting savings in associated construction costs.
In evaluating fire safety of laminated wood structures, two types of fire propagation must be considered: flame spread, and fire resistance.
Flame Spread--a measurement of the maximum extension of the flame on a sample--is a property of the surface material once fire has started, not of the structure. (Under test conditions, it is a ratio, not inches, versus the arbitrary standards). Flame spread is an indication of how fast fire can extend over a room's surface, but says little about the third dimension of fire spread, i.e. the depth of fire penetration through the substrate. Most of the fire retardant coatings influence only the flame spread rate, that is, they function to lower the lateral extension of the flaming surface per unit of time, but have little influence on the "Burn-Through Rate", since the coatings are a surface application. One exception to this statement is the disclosure in the Ellis Patent Application referred to above, in which the surface coating prevents the ignition of the substrate so that a carbonized insulative char forms under the coating--pyrolysis without flame--and in an area restricted to the immediate zone of the initial fire.
Fire Endurance (or, Resistance) is "The resistance against fire penetrating a wall, floor, or roof, either directly or through a high rate of heat transfer that might cause combustible materials to be ignited on the `cold` side of the wall away from the actual fire". In short, it indicates the ability to resist "burn-through" and measures the containment of the fire within a room or building. The standard test for measuring fire resistance is ASTM E-119; and resistance rating is expressed in the hours or the minutes that the construction withstands the test.
The disclosure presented herein defines a novel method for increasing significantly the "fire endurance" of laminated wooden structures, and for decreasing significantly (if not completely preventing) flame spread in these laminated structures by interposing a non-flammable high temperature resistant coating, in a variety of arrangements, between the plies of the laminate, and, optionally, on the surface of the "plywood".